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Chapter 18: Applying Ocean Loading from a Hydrodynamic Analysis

Pressures and motion effects of a rigid body modeled by hydrodynamic analysis can be imported for further analysis using ocean commands. For this type of analysis, the wave type in the ocean wave OCDATA command should be set to diffracted wave (KWAVE = 8). The hydrodynamic data is read in via the OCREAD command.

This topic does not describe wave-theory-derived ocean loading (implemented via KWAVE = 0 through 7 on the OCDATA command).

The following topics related to using hydrodynamic analysis data to apply ocean loading are available:

18.1.How Hydrodynamic Analysis Data Is Used

18.2.Hydrodynamic Load Transfer with Forward Speed

18.3.Hydrodynamic Data File Format

18.4.Example Analysis Using Results from a Hydrodynamic Diffraction Analysis

18.1. How Hydrodynamic Analysis Data Is Used

Pressures and motions due to the waves, current, and forward speed at a given phase are extracted from a hydrodynamic data file. The hydrodynamic data file information is translated either into surface element pressures or into line element pressures based on Morison’s equation.

The hydrodynamic data (.ahd) file must be prepared in a specific format for it to be read during the ocean loading procedure. If the Hydrodynamic Diffraction System (AQWA) has been used for the hydrodynamic diffraction/radiation analysis, a utility program is available to translate the results database into this format automatically.

To apply the ocean loads, an ocean environment and loading data need to be defined using the ocean family of commands (specifically, OCTYPE, OCZONE, OCDATA, and OCTABLE).

For line elements (BEAM188, BEAM189, PIPE288, PIPE289, and LINK180), the ocean loading is applied automatically if it is defined.

For shells or solids, the hydrodynamic loading is applied via SURF154 elements with KEYOPT(8) = KEY, where KEY activates or deactivates ocean effects. If KEYOPT(8) = 0, the ocean load is not applied to SURF154 elements even if the ocean environment exists.

For loads applied to shells or solids only, the hydrodynamic load components are included. Hydrostatic loading can be included on SURF154 using face 4 with PL = -(gravity acceleration) * (water density) and by setting KEYOPT(6) = 1. For loads applied to pipe elements, hydrostatic loading is included automatically.

It is not necessary for the hydrodynamic and structural meshes to be identical, and the loading is mapped automatically from one to the other; however, it is assumed that the hydrodynamic axis system is identical to that used for the structural analysis. Small displacements are assumed in the load-mapping process, and mapping may not be accurate if this condition is not met.